1. Field of the Invention
The present invention is directed to the production of sheets of ultra high molecular weight polyethylene in widths preferably greater than about 60 cm, by ram extrusion.
2. Background Art
Conventional polyethylene polymers with which many are familiar, i.e. low and high density polyethylene, are waxy solids not known for exceptional strength properties. Also, due to their relative softness, while generally lubricious, they are easily abraded. Ultra high molecular weight polyethylene, “UHMWPE”, on the other hand, is an exceptionally strong and durable polymer totally unlike its poorer relatives.
UHMWPE is prepared by special polymerization processes in which the polyethylene chains acquire an exceptionally high molecular weight, typically having number average molecular weights of from 1.5×106 to 1×107 daltons, supplied as a resin powder. UHMWPE also includes those polymers defined in ASTM D4020-05; D6712-01; and ISO 11542-2. While generally homopolymeric in nature, UHMWPE also includes copolymers having limited amounts of other copolymerizable comonomers. In the case of copolymer UHMWPE, the aforementioned ASTM and ISO requirements should be met. It is their very long polymer chains which make these polymers unique. However, this same feature is also the cause of major processing problems. While ordinary polyethylene can be melt extruded, even polyethylene of very high molecular weight, attempts to melt extrude UHMWPE have been largely unsuccessful, despite much research in this area, and virtually all UHMWPE products are produced by compression molding or ram extrusion, as indicated by U.S. Pat. No. 5,286,576, which indicates that processing methods applicable to conventional thermoplastics, such as continuous extrusion, calendaring, and injection molding, are generally inapplicable to UHMWPE.
Thus, for example, U.S. Pat. No. 5,422,061 discloses the manufacture of sliding members by screw extrusion of UHMWPE from the melt. However, for the process to work, mixtures of UHMWPE, lower molecular weight polyethylene (“PE”), and lubricants must be employed. Considerable loss in mechanical properties thus results due to the dilution of the UHMWPE with lower molecular weight species. Further losses in properties are caused by shear degradation in the extruder.
U.S. Pat. No. 5,399,308 discloses melt extrusion through a die whose final cross-section is considerably reduced as compared with the diameter of the extruder, and wherein a very low elongational velocity gradient is maintained. However, despite these requirements, only very high molecular weight PE and UHMWPE in the lowest molecular weight range, about 1.5×106, are useful. U.S. Pat. No. 5,449,484 discloses a screw geometry for a single screw extruder said to be useable with PE resins having molecular weights greater than 1×106. However, large profiles cannot be made using such a melt extrusion process.
The problems associated with processing of UHMWPE are due in part to the gel-like nature of the polymer above its crystalline melt temperature, roughly 135° C.-150° C. While ordinary polyethylene is a viscous, molasses-consistency liquid at such temperatures, UHMWPE is a swollen gel which has an extremely high viscosity, and which exerts exceptional frictional stress against the walls of extruders and the like. The problems associated with UHMWPE are described quite clearly by U.S. Pat. Nos. 3,883,631 and 3,887,319. For the reasons discussed therein, ram extrusion and compression molding have thus far been the only processes of producing UHMWPE products which are used extensively.
In compression molding, UHMWPE powder is introduced into a coffin-like mold of very thick section. A thick cover which fits within the mold cavity is then placed on top of the powder, and the whole assembly is heated to above the crystalline melt temperature while being compressed to very high pressure. The molds are then slowly and uniformly cooled, and the product, generally in the form of a thick slab, is demolded. For thin stock, for example of 1 cm to 3 cm thickness, the thick slabs are skived or “planed” to produce thin stock. The skiving process requires an extra process step, and can result in a product with end curl. As can be seen, compression molding is a cost-intensive and labor intensive method of producing UHMWPE products. However, it is essentially the only process which can be used to make panels or sheets of large width, and is thus still a much used process.
A continuous process for the production of thin gauge UHMWPE sheet has been used by Crown Plastics, Harrison, Ohio, U.S.A. In this process a roller belt press previously manufactured by Hoover Ball and Bearing Co. as the Lam-N-Hard laminator, and as described for use in wood lamination by Tarkow, et al., “Surface Densification of Wood,” FOREST PRODUCTS JOURNAL, 18(a): 104-107, is used to consolidate UHMWPE powder. However, the process thus far has been limited to thin sheets with a maximum thickness of 3-4 mm and relatively narrow widths. Only recently has a 24 inch wide (0.6 m) sheet been produced by this method, and it is not believed to be possible to produce wider sheets due to the high pressures involved.
Ram extrusion is a unique process which is considerably different from melt extrusion. Ram extrusion may be illustrated with reference to U.S. Pat. Nos. 3,883,631; 3,887,319; and 4,145,175. Despite the fact that the ram extrusion technology disclosed in these references is more than 25 years old, there has been only incremental change in ram extrusion processes since that time.
The overall process may be described in relation to FIG. 1 which shows schematically, in cross-section, a simple ram extrusion machine for production of a UHMWPE rod. The ram extrusion machine consists of very thick section steel member 2 having a through channel 3 into one end of which is received ram 4. UHMWPE powder 5 flows gravitationally into channel 3 from hopper 6. The ram then travels to the left, compressing the powder in the channel, which is now die channel 7. This sequence is repeated every few minutes. Die channel 7 is heated by heaters 8 which surround the die, and heats the resin particles to a relatively high temperature, for example between 350° F. and 500° F. (177° C. and 260° C.). Temperatures in excess of 500° F. (260° C.) are generally avoided, since the polymer rapidly oxidizes above this temperature. Oxidized polymer exerts yet more friction with the die, and due to the oxidation, products have reduced physical characteristics. The ram exerts a pressure up to several thousand lb/in2, and consolidates the heated, gel-like particles of UHMWPE. The UHMWPE rod 9 exits the die at discrete intervals and at this stage the rod is hot, higher than the crystalline melt temperature, and relatively translucent. However, at some distance from the die face, the polymer has crystallized to an opaque solid.
Ram extrusion has been used to produce round profiles of relatively large diameter, e.g. 300 mm, and also tubing, small profiles of complex shape, and “boards” having a width of up to about 660 mm, and thicknesses of, e.g. 100 mm. However, such boards are far from flat as produced. If flat boards or thin stock are needed, the boards are skived. Because of the high friction within the die, and consequently the very high pressures involved, the ram, even though made of very high strength steel, may buckle. This is particularly so in parts whose cross-section is quite asymmetric, and even more so in parts having a substantially rectangular cross-section of high aspect ratio. For example, a ram for producing a board of 1 cm×30 cm rectangular cross-section may exhibit buckling, whereas a ram for a thicker board, for example 10 cm×30 cm, will have virtually no problem in this respect at all. Not only can buckling be destructive of the ram, but the distorted ram may scrape the die walls, introducing metal particles into the product and altering die geometry.
It would be highly desirable to employ ram extrusion to produce sheets and panels of large width, for example 1 to 3 meters in width, and of a range of thicknesses, in particular, in standard thicknesses which can be sold as is, without skiving. However, attempts to use the ram extrusion process to prepare such sheets and panels meeting the necessary product standards have been largely futile. The failure of others in the past can be attributed to a number of factors. First, the nature of UHMWPE is such that there is considerable volume contraction upon both cooling and crystallization. Differential cooling or crystallization generates internal stress, as does also differing degrees of polymer orientation. In small profiles or even larger products which are relatively symmetrical, these problems are minimal, or are to a degree self-cancelling. However, in large widths, these problems manifest themselves as undesirable “product characteristics” such as thickness variations, bowing, warping, surface fracture, surface irregularities, “walking”, edge waviness, etc. The larger the width of the product, the more difficult is the control of such defects.
Moreover, the ram extrusion apparatus itself also has severe shortcomings. The large top and bottom surface areas associated with a slit die, coupled with the large internal pressure, create forces which are very difficult to control. A slit die of 1 cm height and 1 m width, and of 0.5 m length may experience a force of 2.1·106 N or more on each half of the die depending upon the internal pressure, which is always high. The restraining bolts in this case will have to bear almost 4.4·106 N of force. Even when such a die is constructed of high strength stainless steel of, for example, a 10 cm thickness on each side, the die will deflect so much due to the internal pressure that a board with pronounced greater thickness in the middle as opposed to the edges results. Dies of this size and construction will also rupture, unless supported by massive restraining structures located exterior to the die.
Unlike symmetrical profiles such as rods, tubes, square stock, or small irregular profiles, the large surface area and high aspect ratio of sheets and panels causes them to distort upon cooling below the crystalline melt temperature external to the die. Differences in the rate of cooling and crystallization can cause warping, bowing, thickness and surface irregularities, and the like. Such defects would then require minimally, shaving of the surface and machining to size. However, some defects, for example warp and bow, may be impossible to remove.
For all the above reasons, it has been considered impossible to produce wide sheets and panels of a quality which is commercially acceptable, by ram extrusion.